Tests: find_file restore PATH env variable after test runs

Tests/RunCMake/find_file/FromPrefixPath.cmake

@@ -1,3 +1,4 @@
+set(ENV_PATH "$ENV{PATH}")
 set(ENV{PATH} "")
 foreach(path "/does_not_exist" "/include" "")
   unset(PrefixInPATH_File CACHE)
@@ -15,3 +16,4 @@ foreach(path "/does_not_exist" "/include" "")
   message(STATUS "PrefixInPATH_File='${PrefixInPATH_File}'")
 endforeach()
 set(CMAKE_FIND_USE_SYSTEM_ENVIRONMENT_PATH ON)
+set(ENV{PATH} "${ENV_PATH}")